Myogenesis in vitro has been much studied as a model of cellular differentiation. The formation of multinucleated non-proliferating, electrically active and contractile myotubes by fusion of undifferentiated, proliferating myoblasts is accompanied by the appearance of muscle specific proteins, such as creatine kinase and myosin, and by the arrest of DNA synthesis. Thus, morphological and physiological changes find a counterpart in a series of biochemical events even though the mechanism(s) responsible for these changes in gene expression is (are) completely unknown.